δ13C values of organic carbon in cropland and forest soils in Japan

Abstract

The carbon content and natural 13C abundance (δ13C) of soil organic carbon were determined for Japanese cropland and nearby forest soils by continuous flow CN analysis with mass spectrometry. The δ13C values of organic carbon from cropland soils in the Hokkaido District ranged between −27 and −24‰, suggesting that soil organic carbon originated mainly from C3 plants, while the δ13C values from those in Main Island, Shikoku, and Kyushu Districts ranged between −25 and −17‰, indicating that up to 70% of soil organic carbon originated from C4 plants (presumably Miscanthus sinensis). The δ13C values of organic carbon from Imaichi and Tanegashima forest soils showed that carbon was largely of C3 plant origin in the surface soils but in deeper horizons the values were strongly affected by the organic carbon of C4 plant origin. Sugarcane cultivation for 36 y in Tanegashima cropland soils had resulted in the increase of the soil δ13C values to around −15‰. In Shizuoka Prefecture, the δ13C values of soil organic carbon of non-volcanic ash Andosols under forests at Iwata and Makinohara were within the ranges of C3 plant origin. The values in volcanic ash Andosols at Fuji indicated that the soil carbon in the surface horizons mainly originated from C3 plants while that in deep soil horizons largely from C4 plants. Tea cultivation for more than 40 y had led to a decrease of the δ13C values at Fuji by 2-4‰. In the Miyako Island sugarcane fields, the δ13C values of the plow layers were apparently affected by sugarcane, a C4 plant species, compared to the subsoils where a large fraction of carbon originated from C3 forest plants. The shift from forest to sugarcane cultivation markedly decreased the carbon content and increased the δ13C values of soil organic carbon from −27-−25‰ to −17-−14‰ within 25 y.